SPR is well-known for the detection of chemical species. SPR may be achieved by using the evanescent wave which is generated when a TM-polarised (or p-polarised) light beam is totally internally reflected at the interface between a dielectric medium, e.g. glass, and a thin layer of metal. Any TE-polarised (or s-polarised) component of the radiation cannot excite SPR by the process of total internal reflection and in conventional SPR such components are not employed. The technique is described by Lieberg et al in Sensors and Actuators, 4, 299.
The basis for the application of SPR to sensing is the fact that the oscillation of the surface-plasma of free electrons which exists at a metal-dielectric boundary is affected by the refractive index of the material adjacent to the metal surface. Resonance occurs when the angle of incidence of the radiation has a particular value, and this value is dependent on the refractive index of the material adjacent to the metal. Thus, changes in this refractive index give rise to changes in the angle at which resonance occurs.
A problem which occurs with known SPR devices is that resonance is detected as a reduction in the intensity of the reflected light. This means that the electronic gain of the detector, or the light level from the source, can only be set with respect to the bright background to prevent electronic saturation away from resonance. Small changes in intensity at resonance are difficult to amplify independently for measurement.
We have now devised an SPR device in which resonance is detected as an increase in light intensity. This enables the electronic gain of the detector to be set to suit the strength of the resonance, with associated improvements in sensitivity and measurement accuracy.